The present invention relates to spring-powered air weapons or airguns in which the spring consists of a sealed gas charge as disclosed in GB-B-2084704. The inventors of the subject invention, who are also responsible for the invention identified above and have been responsible for a number of other highly beneficial and successful inventions (e.g. U.S. Pat. Nos. 4,771758 and 4,850,329) in the field of spring-operated airgun power systems and affecting, amongst other things, airgun efficiency, have been making a range of airguns incorporating sealed gas springs for many years. Their products are sold under the Trade Mark "THEOBEN".
Although there are many different systems for powering airguns, such as those involving either precharged tanks of compressed air at pressures of up to about 200 bar (20 MPa) or containers of liquified carbon dioxide which will boil off to produce a substantially constant pressure of about 60 bar (6 MPa), the most popular system by far is one in which the airgun incorporates a self-contained energy storage system whereby a single, manual, cocking stroke will create a quantity of stored energy which can subsequently be released when desired, by means of a trigger mechanism, to discharge a projectile. It will readily be appreciated that, if the airgun is to be fired reasonably frequently by persons of average strength, the single manual stroke referred to will not and cannot involve very large amounts of energy. Or, to put it another way, the gross energy input per shot is perforce quite modest and, therefore, the overall efficiency of the airgun power system, i.e. the efficiency with which the work done in cocking the airgun is converted into kinetic energy in the projectile when released, is of considerable importance. Unlike cartridge firearms, single-stroke airguns have only a modest amount of energy input available to start with so, unless the process of conversion is reasonably efficient, the power of the airgun will be extremely low.
DE-C-1553962 discloses an air weapon in which the energy projecting the pellet is achieved through a gas spring. However, the constructional details of the gas spring are not discussed; the specification merely refers to a gas spring consisting of a spring cylinder and a displacement body, such springs being known per se. Later, it is suggested that the displacement body might consist of a cylindrical piston rod. In the absence of any specific constructional details, it might be assumed that the gas spring is of a conventional design and consists of a cylinder, a piston slidingly sealed to the inside wall of the cylinder and a piston rod rigidly attached to the piston. The variable-volume sealed space between the piston crown and the inside on the cylinder defines the working gas chamber which will be under pressure. Behind the piston there will be a rear chamber which must be open to allow air to escape when the cylinder shoots forward upon firing. The cylinder itself, of course, constitutes the piston which slides within the airgun compression chamber behind the pellet. Such a construction would exhibit a very considerable compression ratio in moving the piston crown in its sliding cylinder from the uncocked to the cocked position.
The volume occupied by the gas in the sealed working gas chamber is reduced to a very small value when the system is cocked whereas the volume of the sealed working gas chamber represents almost the entire volume of the cylinder itself when the system is in the uncocked position. Thus, the compression ratio in this gas spring may be as high as 8 or 10. There are two practical effects in having a high compression ratio. Firstly, the effort required to move the piston from the uncocked to the cocked position increases markedly as the working gas is compressed.
The second disadvantage in having a high compression ratio is that the force which causes the piston to accelerate down the compression chamber when released by the trigger is far from uniform. At high power levels, the flow of hot, compresseed air produced by this non-uniform acceleration appears to be likely to deform the pellet and thereby impair accuracy.
It will also be noted that DE-C-1553962 does not contemplate any means of varying the uncocked gas pressure in any given unit. Nevertheless, changes in performance are allegdly to be achieved by changing the entire gas spring assembly.
FIGS. 1 and 2 are simplified illustrations of an airgun containing a sealed gas spring in accordance with GB-B-2,084,704. FIG. 1 shows the airgun in the cocked condition, i.e. with the main piston 28 held in its rear-most position by a trigger mechanism 60. The airgun consists of a barrel 10 whose breech communicates with a compression chamber 25 via a transfer port 24. The main cylinder 26 contains a piston 28 which consists of a hollow tube 30 sealed at one end by a piston crown 32. The tube 30 of the piston 28 is a sliding fit over a static cylinder (or "dummy piston") 36 with a seal between the inner bore of tube 30 and the outer bore 38 of dummy piston 36. Thus a sealed space of 52 of variable volume is created which communicates with a sealed space 52A via the bore 44 in the dummy piston 36.
FIG. 2 shows the same airgun, further simplified, in the fired condition. Thus the piston 28 has moved to the left so as to compress the air in the compression chamber 25, forcing it through the transfer port 24 and out of the barrel, taking the projectile with it. It will be appreciated that the sealed, variable volume chamber 52 + 52A can be pre-charged with gas at a pressure substantially higher than ambient. In practice, a pressure of about 20 bar (2 MPa) has been found to suit most applications. Clearly the pressure in the sealed chamber will rise pro rata to the reduction in its volume as the piston is forced back during the cocking stroke. Typically the volume of 52 + 52A when fully cocked will be about 2/3rds of its volume when uncocked, i.e. a compression ratio of approximately 1.5:1. The pressure in the sealed chamber will rise in inverse proportion to the reduction in volume and is thus likely to be of the order of 30 bar (3 MPa) when the airgun is cocked.
A potential disadvantage of the sealed gas spring system without the present invention, is that it is, in effect, a variable-rate spring for, as the volume decreases during the cocking stroke and the pressure rises, so the additional force required to move the piston a further given distance also increases, whereas a uniform metal coil spring should have a substantially constant spring rate and so the additional cocking force per unit of distance will remain substantially constant through the travel of the piston. This disadvantage can, however, be ameliorated to some extent by skilful arrangement of the pivoting geometry of the cocking mechanism so as to achieve an increasing mechanical advantage during the stroke.
It may be helpful to give some indication of the levels of efficiency involved. A fairly typical conventional airgun, incorporating a metal coil spring in place of the sealed gas spring of the Theoben System, may have an overall efficiency in the range of 10% to 15%. Existing Theoben air riffles, incorporating the inventions of the present inventors as identified above but without the subject invention, can reach efficiencies of up to about 20%. By way of contrast, a multi-stroke pneumatic airgun, e.e. an airgun incorporating a self-contained pump which may be operated many times to compress increasingly a charge of air which, generally, will all be substantially released to force the projectile out of the barrel when the trigger is operated, may have an overall efficiency of only 1 or 2%.
From all the above it will be appreciated that the search for energy efficiency in a single-stroke spring airguns has been under way ever since this class of airgun became popular in the latter part of the 19th century. Certainly it has been a major goal of the present inventors for the past decade and one at which they have already been proved to be extremely successful.
Nevertheless, the inventors have, on some occasions, been unable to achieve the desired power output when converting, for example, another manufacturer's rifle with a relatively small compression chamber capacity, to their sealed gas spring system. In addition, when attempting to produce very high power outputs from their own rifles, increasing the pre-cocked pressure in the sealed gas chamber has repeatedly been found to have only a limited and non-linear effect. Thus, for any given set of compression chamber dimensions, increasing the pre-cocked pressure from its normal level, in small, uniform steps, will have a general tendency to increase the performance of the rifle in a corresponding series of steps which grow smaller and smaller and eventually decrease to nothing. During this process the firing action of the rifle will tend to become increasingly harsh and unpleasant. If the pressure is increased even further, the cocking effort will continue to increase very noticeably and yet the kinetic energy transferred to the projectile may actually decrease. Thus, the overall efficiency with which the cocking effort is converted into kinetic energy in the projectile will drop rapidly with increasing pressure.
This general pattern tends to occur in coil spring airguns as well, in that if more and more powerful springs are fitted, the cocking effort increases pro-rata, the gun becomes harsh to fire and small initial power increases rapidly diminish with further increases in spring strength until they cease altogether and the power may even start to decrease.
It is believed that one of the principal reasons why the efficiency of an airgun built in accordance with GB-B-2,084,704 should start to decrease once the pre-cocked pressure in the sealed chamber is increased past a given point, is probably because the higher pressure increases the frictional drag of the seal which usually consists of one or more O-rings or other seals, mounted in either the inner bore of the piston and sliding on the outer surface of the dummy piston, or in the outer bore of the dummy piston and sliding on the inner surface of the piston, faster than the increased pressure increases the forces tending to accelerate the piston down the compression chamber. It is a general rule that the higher the pressure acting on an O-ring seal, the greater will the force with which the O-ring grips the member on which it is sliding. Other things being equal, the greater the diameter of the circle of contact between the O-ring and the surface on which it is sliding, the greater the frictional drag, since the length of the contact surface between the O-ring and the surface in sliding contact with it, will be directly proportional to the diameter of the O-ring.
It is highly likely that there are various other complex factors involved in the overall reduction in efficiency, probably including the rapid heating of the air in the compression chamber during the firing stroke and the flow dynamics of this hot, compressed air through the transfer port and into the barrel. Nevertheless the subject invention has produced substantial benefits without further development of the compression chamber or transfer port.